Chandrayan-2 Mission

[Mort Walker]

114 km x 18072 km -achieved
vs
118 km x 18078 km -planned

WOW !!!!!

Most excellent!!!!

[prasannasimha]

114 km x 18072 km -achieved
vs
118 km x 18078 km -planned

WOW !!!!!

Most excellent!!!!

That's a sigma of 4.505 for apogee

[prasannasimha]

Talked to an actual space engineer and the three body problem and involvement of Lagrangian points are not used except for actual missions to Lagrangian points. The ellipse assumption is enough for modeling for lunar missions

[disha]

There were a couple of occasions when Dr Sivan was asked about how the landing date was maintained even though the launch date slipped by a week.
It was my understanding that this would be done by reducing the number of orbits around the moon after LOI was achieved. But Dr Sivan did not mention that at all, instead he talked about the time of TLI being adjusted. I wonder what is the real situation.
Another question I have is regarding the orbits. If I am not mistaken, there is not much inclination between the equatorial planes of earth and moon. Since we need to be in polar orbit around the moon, if we launch into polar orbit around the earth and then do the transfer to moon, we will arrive very close to polar orbit around the moon....

RonyKJ, in any of such major projects - buffers are built in. This is part of project management. Think about it, C-2 was planned around 2005, negotiated around 2007 and approved in 2008. It also got delayed due to Russians pulling out.

In this case, a simple explanation is that ISRO had identified a launch window and as long as that window was maintained the rest of the dependencies come through. For example, an additional 7 days would have given ISRO time to additionally map and analyze the landing site. Also check out some maneouvers and if there are contra-indications to that they can probably evolve additional work arounds. That buffer is lost. Hence the time of reaction is shortened.

From this point on wards, it is basically seat of the pant flying. That is until now, that is until LOI algorithms used were already validated by C-1 (almost). So it was only a question of existing & tested systems firing correctly. From now on, it is altogether uncharted. That is identify the landing area and then make sure the lander algorithms work correctly. Assuming the algorithms work correctly, the new systems fire correctly.

Coming to your question, think of it as follows:

1. Imagine space as a fabric. A very taut fabric but flexible as well without tearing. Place Earth as a giant marble, and it will cause a deformation on the fabric. This is Earth's gravity well. At the edge of the Earth's gravity well on the inside and towards the top sits Moon. It has its own gravity well. Though not as deep and wide as Earth.

2. C-2 was *not* launched on the polar orbit. It was launched at an inclination to the equatorial orbit. Currently think of Earth is tilted backward in relation to Moon on the equatorial plane. We do not need significant tilt, just some tilt. Also moon is not that much tilted.

3. Now with subsequent orbit raising maneuvers, C-2 was sling shot across to the moon, just so much that its path was over the top of the moon. That is on Moon's polar.

So what is LOI. At this point, the LOI burn is to ensure that it falls into Moons' gravity well. Thus the change in orbital planes.

Added later,

Just saw parts of Dr. Sivan's press conference on LOI (and a must watch, audio quality is though bad). It clarified couple of things,

1. Minor changes in the trajectory ensured that C-2 is able to make its Sep 7 date. So my take is that the LOI date was delayed but the rest of the mission parameters were already accounted for.

2. Everything was very precise. Including the firiing. Say an additional 10 cms/sec velocity gain during TLI would have resulted in a 7* inclination in polar orbit around moon and thus rendered the mission useless.

[disha]

And here is an animated gif on twitter that shows the capture very well

https://twitter.com/SwamiGeetika/status ... 1085120512

[SwamyG]

And here is an animated gif on twitter that shows the capture very well

https://twitter.com/SwamiGeetika/status ... 1085120512

Super gif. It feels like some movie scene.

[SSSalvi]

@Rony
If Earth orbit was polar, then C2 would have traveled northwards or southwards whereas the moon is in almost equatorial orbit.
So initial orbit was near to ecliptic but slightly adjusted to match the intended LOI point taking into consideration the liberation of moon orbit.
----
Final descent of lander is fixed by the start of lunar day at ( pre decided ) South Pole site.
Refer my post on page 13 about next lunar Dawn (Sept 5th ) and eventide (Sept 19 th )near lunar South Pole.
So it makes no difference as to when you Start the mission.
If you enter moon early ( if launch was as originally planned ) then dwell some more time orbiting moon till desired descent moment.

Depending on the battery condition before solar recharge you could land on surface a few days prior to dawn and stay in sleep mode until the Sunlight. ( Sorry, no birds chirping before sunrise)

[juvva]

Inching closer to the lunar surface:

https://www.isro.gov.in/chandrayaan2-latest-updates

August 21, 2019

Second Lunar bound orbit maneuver for Chandrayaan-2 spacecraft was performed successfully today (August 21, 2019) beginning at 1250 hrs IST as planned, using the onboard propulsion system. The duration of the maneuver was 1228 seconds. The orbit achieved is 118 km x 4412 km.
All spacecraft parameters are normal.
The next Lunar bound orbit maneuver is scheduled on August 28, 2019 between 0530 - 0630 hrs IST.

[juvva]

^
118km x 4412 km : achieved
vs
121km x 4303 km : planned

[ArjunPandit]

how do they determine distance at such large distances esp when everything is moving?

[SSSalvi]

how do they determine distance at such large distances esp when everything is moving?

By dopppler shift measurement. It is an involved theory.
At least l can't explain in simple terms.

Regarding accuracy, in 1975 , with the then technology, which was contained in the size of 5 racks of 6 ft height , the ranging equipment ( that is the technical name ) could measure geosync satellite ( 36000 kms altitude ) position with an accuracy of 1 mtr.

Now this function can be achieved with a laptop.

[Varoon Shekhar]

Excellent performance so far! Accuracy is wonderful. Is there any reason that more pics haven't been released after those very fine ones about 10 days ago? Oh well, we'll see plenty in the months ahead!

[ArjunPandit]

Everytime i read about ellipse, i am mentaly remember the genius of ramanujan on how he came up with the approximation on the perimeter of ellipse..and how such a seemingly simple problem does not have a simple solution

[Indranil]

how do they determine distance at such large distances esp when everything is moving?

By dopppler shift measurement. It is an involved theory.
At least l can't explain in simple terms.

Let me give it a shot. It is probably more layman than what you considered layman.

1. By measuring the strength of the reflected radar signals, one can accurately determine the position of an object relative to the earth
2. By measuring the rate of change of this strength, one can accurately determine the velocity vector of the object relative to the earth
3. Since the moon’s position and relative velocity to earth are known, it is easy to now calculate the relative position and velocity of the object relative to the moon
4. Moon’s mass is known. If you neglect the gravitational effects of other celestial objects*, then the orbit of the object around the moon can be very easily calculated. Just refer to Kepler.

*The relative position and velocity of other celestial objects and the moon is very similar to that of a near-moon-satellite.

[ldev]

SSSalvi,

The table below gives the planned orbit raising schedule for Chandraayan 2 (already achieved)

Chandrayaan 2 Raising Schedule:

24 JUL 19: 14:00-15:30 241.5 x 45,162 Km
26 JUL 19: 01:00-02:00 262.9 x 54,848
29 JUL 19: 14:30-15:30 281.6 x 71,341
02 AUG 19: 14:00-15:00 262.1 x 89,743
06 AUG 19: 14:30-15:30 233.2 x 143,953

I have two questions:

1. How long does it take complete 1 orbit at each apogee/perigee distance?
2. And the real question is why wait for days between each orbit raising maneuver? Why not do the orbit raising in successive orbits? This will cut down the transit time? Similarly on being inserted into moon orbit, why wait for days between each orbit lowering retro firing?

Thanks

[Cain Marko]

Is there any cool video/simulation that shows the transfer to lunar orbit?

[SriKumar]

By dopppler shift measurement. It is an involved theory.
At least l can't explain in simple terms.

Let me give it a shot. It is probably more layman than what you considered layman.

1. By measuring the strength of the reflected radar signals, one can accurately determine the position of an object relative to the earth
2. By measuring the rate of change of this strength, one can accurately determine the velocity vector of the object relative to the earth
3. Since the moon’s position and relative velocity to earth are known, it is easy to now calculate the relative position and velocity of the object relative to the moon
4. Moon’s mass is known. If you neglect the gravitational effects of other celestial objects*, then the orbit of the object around the moon can be very easily calculated. Just refer to Kepler.

Couple of comments. SSSalvi's comment refers to Doppler. So it must be a frequency-based technique that he is referring to; your explanation is based on the strength (amplitude). Two different approaches.
The original question by arjunpandit seems to refer to measurement of distances far away, and I think he is referring to the lunar orbit perigee/apogee of CY2. I dont believe earth-based radar would be used to estimate the lunar orbit of CY2 when circling the moon (even though your method would theoretically work). Reason being that its orbit and velocity values need to be local to moon as its center (this is what is reported by ISRO). As to how its lunar orbit perigee, apogee is measured, I have to believe it happens on board the craft (and not from an outside agency). I assume it would be onboard radar or laser to estimate its closest, farthest point from lunar surface, and several other intermediate points in the orbit, then compute/interpolate a mathematical orbit based on the mass of moon and CY's own velocity, compute the center of that orbit and then correct/modify its perigee/apogee in relation to the computed center of orbit. I think this might be the approach. Note that a predicted orbit was published even before the craft was launched, so the orbit would have to be computed based on parameters known before launch.

The reason I mention re-computing the perigee/apogee is because moon's surface is uneven, so there is not a reference surface. Perhaps this is why it needs to run repeated orbits...to compute a correct center of orbit (in addition to mapping the surface for a good landing site). My theory anyway.

[sudarshan]

Let me give it a shot. It is probably more layman than what you considered layman.

1. By measuring the strength of the reflected radar signals, one can accurately determine the position of an object relative to the earth
2. By measuring the rate of change of this strength, one can accurately determine the velocity vector of the object relative to the earth
3. Since the moon’s position and relative velocity to earth are known, it is easy to now calculate the relative position and velocity of the object relative to the moon
4. Moon’s mass is known. If you neglect the gravitational effects of other celestial objects*, then the orbit of the object around the moon can be very easily calculated. Just refer to Kepler.

I see that SriKumar beat me to it, but yes, Doppler is based on frequency, not amplitude. Just theoretically speaking, I have no clue if this is what actually happens in practice (for all I know, whatever I'm prattling below could be straight from a Madrasa course - but what the heck, let me stick my neck out a little, willing to be corrected on the below):

1. You measure distance by time delay - how long does the laser signal take to bounce back from the object? Or how long does a laser signal from CY-2 take to reach the earth (you can modulate the signal to encode time information, and if there is an on-board clock synchronized with time somewhere on earth, then you get a pretty accurate estimate of transit time).

2. You measure velocity in two stages

2.a. First the velocity in the tangential direction from earth - angular shifts, parallax
2.b. Then the velocity in the radial direction from the earth - this is where the "Doppler" part comes in - basically, you know the true frequency that the laser beam is supposed to have, and you also measure the apparent frequency. The shift gives you the radial velocity - this is the definition of the Doppler effect, after all.
2.c. Combine 2.a. and 2.b. vectorially, you have the 3-d velocity.

The problem with the amplitude-based measurement is - unknown (and variable) losses due to scattering along the way or other effects (to be honest, you can also have refractive index changes on the way which mess with the speed of the laser beam). But the second problem is with the rate of change. As indicated earlier - the velocity of CY-2 in a 100 km circular orbit around the moon would be like 1.7 km/s. The earth-moon distance is ~400,000 km. Signal strength diminishes as square of distance (take to-and-fro distance, and you have twice the distance, so square of distance would do a factor of four). So the rate of change of the signal amplitude due to the velocity of the craft would be very hard to make out. And if the velocity is entirely tangential, then there is even less rate of change than if the velocity is radial. Too problematic this way. Doppler and parallax would be better.

But maybe all this is unnecessary, the above assumes earth-based measurements. It would probably be more practical for CY-2 to do its own moon-based measurement (since it is much closer to the moon). So - parallax and Doppler based on the moon - gives velocity relative to the moon (feeds into perigee/ apogee calculations). And distance from moon from time delays of laser beam fired from CY-2 at the moon and back. I think this would be much more practical, if there was the requisite hardware and computing power on board. Then just beam back the info to the earth.

Again, all this is my own guess, willing to be corrected on any or all of the above.

[SSSalvi]

^
To give simple analogy:
In preliminary physics we had problem of Echo ... I receive echo after 1 second so what is the distance between me and the mountain?
That gives distance in stationary situation.
If you are moving then do the measurement continuously at equal intervals .. you will get the object receding speed by measuring the distance change in each occasion.

That is with audio freq .. if you use RF freq then you can do same measurements more frequently because the RF speed is higher and so determine the speed more accurately.

You do a similar process but you measure not the echo delay you also measure doppler ( Again primary arithmetic : Train whistle ( La Pakeeza ) changes frequency with speed )

You change terminology from Distance to Range and Speed to Range Rate

and you get the modern equipment called R&RR ( Range and Range Rate ) computer!!


====

SSSalvi,

The table below gives the planned orbit raising schedule for Chandraayan 2 (already achieved)

Chandrayaan 2 Raising Schedule:

24 JUL 19: 14:00-15:30 241.5 x 45,162 Km
26 JUL 19: 01:00-02:00 262.9 x 54,848
29 JUL 19: 14:30-15:30 281.6 x 71,341
02 AUG 19: 14:00-15:00 262.1 x 89,743
06 AUG 19: 14:30-15:30 233.2 x 143,953

I have two questions:

1. How long does it take complete 1 orbit at each apogee/perigee distance?
2. And the real question is why wait for days between each orbit raising maneuver? Why not do the orbit raising in successive orbits? This will cut down the transit time? Similarly on being inserted into moon orbit, why wait for days between each orbit lowering retro firing?

Thanks

Period in Hrs
24 July 13.68586608
26 Jul 17.23179373
29 Jul 24.04596692
2 Aug 31.87325697
6 Aug 59.81055005

Orbit raising can be done in one after another orbits.
But there is ample time
secondly ( and this is technical ) the inclination was slowly changed from 7 deg to about 6 deg and if you want more accuracy then you can do more measurements in each orbit to determine the impulse to be given in next raising.

As far as moon is concerned we are reaching a few days before the appropriate time ( based on the landing site getting the sunlight ) so we have to wait and continue revolving around moon.

Due to moon's uneven gravity you can't remain in a fixed orbit without drifting and that drift changes with orbit geometry. So each designer can have his/her plan of action ( within the goal to be achieved ) of orbit raising.

=== Reproducing the images from my earlier post about Lunar day:


The Lander and the rover have to land at the specified location and .... The Location should be well illuminated and so the whole mission timing is decided with that goal.

First, the Physicist have decided the location ( high plain between craters Manzinus C and Simpelius N, at a latitude of about 70° south ).

Here is an attempt to map the site and its illumination conditions.

Image below shows the Vikram Lander's Planned position on Moon.
Left is the general view and Right side shows the same situation when viewed from the South of Moon.


Next we try to map when this location starts getting sunlight and upto what time.

On 5th Sept 2019 the the Landing site starts getting illuminated as shown in image below.
( Full disc of Moon view on Left and details on Right )


The Locations remains illuminated upto 19th Sept 2019.

[ldev]

^
Period in Hrs
24 July 13.68586608
26 Jul 17.23179373
29 Jul 24.04596692
2 Aug 31.87325697
6 Aug 59.81055005

Orbit raising can be done in one after another orbits.
But there is ample time
secondly ( and this is technical ) the inclination was slowly changed from 7 deg to about 6 deg and if you want more accuracy then you can do more measurements in each orbit to determine the impulse to be given in next raising.

As far as moon is concerned we are reaching a few days before the appropriate time ( based on the landing site getting the sunlight ) so we have to wait and continue revolving around moon.

Due to moon's uneven gravity you can't remain in a fixed orbit without drifting and that drift changes with orbit geometry. So each designer can have his/her plan of action ( within the goal to be achieved ) of orbit raising.]

Thank you. Very helpful. You are the boss for all things satellite/spacecraft related!!

[sudarshan]

^

If you are moving then do the measurement continuously at equal intervals .. you will get the object receding speed by measuring the distance change in each occasion.

That is with audio freq .. if you use RF freq then you can do same measurements more frequently because the RF speed is higher and so determine the speed more accurately.

You do a similar process but you measure not the echo delay you also measure doppler ( Again primary arithmetic : Train whistle ( La Pakeeza ) changes frequency with speed )

Both of these only give you speed towards/ away from the target object (radial speed), right? For tangential speed, you still need parallax?

For perigee/ apogee calculations, you need all the components of the velocity (in addition to distance of course), if I'm not mistaken.

[SSSalvi]

^ True
But with continuous R&RR one can determine the variations in X,Y, Z co-ordinates.
R&RR sessions last several hours if need be .. but at typically an hour or so.
====
Pre-answer to expected corollary

After 21st Aug orbit Lowering : Orbit period is about 28 hours
and after 28th it will be 6.3 Hrs
=====
Just to emphasize that different designers have different options based on resources at hand



3 nations had 3 different paths to reach Mars in the same timeline.

India sarted early because it had to do orbit raising similar to CH2 becuase of launcher limitation.
( Re : BRF Mangalyaan thread page 31 )

[srin]

To get to the Lunar Transfer Trajectory, Chandrayaan-2 gradually increased the apogee using Oberth Effect via LAM firing at the perigee. I understand that.
Now, couple of questions:
But once around the orbit of the moon, to decrease the apogee and get to a more circular orbit, does the LAM fire at the perigee (in reverse direction) or at the apogee ? I suspect it is a reverse direction firing at the apogee, but not too sure.
Also, to do an orbital inclination change to lunar polar orbit, I suspect the LAM firing will happen at the apogee (where the speed is the lowest and direction change is best). Is that right ?

[disha]

Is there any cool video/simulation that shows the transfer to lunar orbit?

Check the animated gif in the twitter link I put it some posts up. It ha LOI.

[srin]

Is there any cool video/simulation that shows the transfer to lunar orbit?

You can also check this one: https://sankara.net/chandrayaan2.html. This was posted on Hacker News

[Varoon Shekhar]

https://www.isro.gov.in/update/22-aug-2 ... 19-1903-ut

Nice first pic of the moon taken by Chandrayaan-2

[SaiK]

wouldn't we be landing on the dark side?

[prasannasimha]

To get to the Lunar Transfer Trajectory, Chandrayaan-2 gradually increased the apogee using Oberth Effect via LAM firing at the perigee. I understand that.
Now, couple of questions:
But once around the orbit of the moon, to decrease the apogee and get to a more circular orbit, does the LAM fire at the perigee (in reverse direction) or at the apogee ? I suspect it is a reverse direction firing at the apogee, but not too sure.
Also, to do an orbital inclination change to lunar polar orbit, I suspect the LAM firing will happen at the apogee (where the speed is the lowest and direction change is best). Is that right ?

Firing at apogee circularizes the orbit
firing reverse at the perigee lowers the apogee (You actually can never make a point burn)

[Varoon Shekhar]

Is that correct, about the recent pictures coming from the Vikram lander? The lander is not described as having an optical camera. It has 3 other type paylods. Do they mean the Orbiter?

[krishGo]

wouldn't we be landing on the dark side?

No! The planned landing site is on the visible side of the moon to south. Check the image posted by SSSalvi sir above.

[Cain Marko]

Is there any cool video/simulation that shows the transfer to lunar orbit?

You can also check this one: https://sankara.net/chandrayaan2.html. This was posted on Hacker News

Is there any cool video/simulation that shows the transfer to lunar orbit?

Check the animated gif in the twitter link I put it some posts up. It ha LOI.

Thank you saars

[sudarshan]

^ True
But with continuous R&RR one can determine the variations in X,Y, Z co-ordinates.
R&RR sessions last several hours if need be .. but at typically an hour or so.
...

Ok, that would work I guess, good old least-squares fitting using some kind of optimization model (heuristic routine), to find the orbit which best fits the observed distance and radial velocity readings over the several hours that data is gathered.

Thanks for the gyaan.

[prasannasimha]

Is that correct, about the recent pictures coming from the Vikram lander? The lander is not described as having an optical camera. It has 3 other type paylods. Do they mean the Orbiter?

No it is from the orbiter

[pravula]

wouldn't we be landing on the dark side?

Dark side is not dark iirc.

[prasannasimha]

Is that correct, about the recent pictures coming from the Vikram lander? The lander is not described as having an optical camera. It has 3 other type paylods. Do they mean the Orbiter?

The images are from the orbiter

[prasannasimha]

Orbital inclination changes are typically done at the ascending or descending nodes or at intersection of current and desired orbit

[juvva]

@Rony
----------
----------
----------
Depending on the battery condition before solar recharge you could land on surface a few days prior to dawn and stay in sleep mode until the Sunlight. ( Sorry, no birds chirping before sunrise)

Landing in the dark before dawn, would be a huge challenge.......

[ldev]

I really hope that ISRO has got high definition cameras in the lander/rover. Does anyone know the specs of these cameras, their resolution? If it is DD like coverage/resolution of their launches, it will be a disaster.

I know data transfer rates are the bottleneck for transmitting high resolution pictures, but still.......

[Indranil]

Let me give it a shot. It is probably more layman than what you considered layman.

1. By measuring the strength of the reflected radar signals, one can accurately determine the position of an object relative to the earth
2. By measuring the rate of change of this strength, one can accurately determine the velocity vector of the object relative to the earth
3. Since the moon’s position and relative velocity to earth are known, it is easy to now calculate the relative position and velocity of the object relative to the moon
4. Moon’s mass is known. If you neglect the gravitational effects of other celestial objects*, then the orbit of the object around the moon can be very easily calculated. Just refer to Kepler.

Couple of comments. SSSalvi's comment refers to Doppler. So it must be a frequency-based technique that he is referring to; your explanation is based on the strength (amplitude). Two different approaches.
The original question by arjunpandit seems to refer to measurement of distances far away, and I think he is referring to the lunar orbit perigee/apogee of CY2. I dont believe earth-based radar would be used to estimate the lunar orbit of CY2 when circling the moon (even though your method would theoretically work). Reason being that its orbit and velocity values need to be local to moon as its center (this is what is reported by ISRO). As to how its lunar orbit perigee, apogee is measured, I have to believe it happens on board the craft (and not from an outside agency). I assume it would be onboard radar or laser to estimate its closest, farthest point from lunar surface, and several other intermediate points in the orbit, then compute/interpolate a mathematical orbit based on the mass of moon and CY's own velocity, compute the center of that orbit and then correct/modify its perigee/apogee in relation to the computed center of orbit. I think this might be the approach. Note that a predicted orbit was published even before the craft was launched, so the orbit would have to be computed based on parameters known before launch.

The reason I mention re-computing the perigee/apogee is because moon's surface is uneven, so there is not a reference surface. Perhaps this is why it needs to run repeated orbits...to compute a correct center of orbit (in addition to mapping the surface for a good landing site). My theory anyway.

1. I don't know if they track based on amplitude or frequency. But both methods are possible, but I will trust Salvi sahab's observation on Doppler being used for terrestrial tracking
2. AFAIK the measurements are done both onboard and terrestrially. There was a GSLV launch (I have forgotten which one, but the PM was present) where the terrestrial tracking went off. The terrestrial tracking is what you see on the launch screen.
3. Actually, I will be very surprised if the onboard navigation system is radar based. The instantaneous velocity and position are known at all times, even during orbit manipulation/maintenance. This is used for automatic LAM engine start and shut-off for orbit raising and station keeping. This was also used in MOM. Rememebr the time when MOM went into eclipse behind Mars. All the station keeping was done automatically.
4. IIRC the onboard navigation system was a highly adapted INS system which was re-calibrated using terrestrial observations periodically. I will find out.

[Indranil]

I really hope that ISRO has got high definition cameras in the lander/rover. Does anyone know the specs of these cameras, their resolution? If it is DD like coverage/resolution of their launches, it will be a disaster.

I know data transfer rates are the bottleneck for transmitting high resolution pictures, but still.......

Is the objective of Chandrayaan our viewing pleasure?